Fig. 7: Mechanism by which the constant bearing geometry of an incidental collision course can avoid a confusion effect in a dense swarm.
From: Raptors avoid the confusion effect by targeting fixed points in dense aerial prey aggregations
A Composite image showing the motion of a hawk and the bat that it catches from the swarm, as seen from a fixed video camera. The trajectories of the hawk (blue points) and the bat that it ultimately captures (black points) are sampled at 0.02 s intervals and are shown connected by the instantaneous line-of-sight from the hawk to the bat (white lines). The composite images of the hawk are sampled from the video at 0.2 s intervals and superimposed on the frame corresponding to the catch; red starburst shows the actual point of capture. Note that the line-of-sight remains approximately parallel as the hawk plunges into the swarm, which is because the hawk is on a collision course with the bat that it catches. B Projected flight velocity (yellow arrows) of the bats tracked in Fig. 1A (black points), as seen by a stationary observer. The projected flight velocity of the highlighted bat (red arrow) is used as a reference in the next panel. C Apparent velocity (cyan arrows) of the same bats, as seen by a moving observer whose translational motion makes the apparent velocity of the highlighted bat (red circle) zero, such that it appears stationary against the distant background. Provided that its range is decreasing simultaneously, this constant bearing geometry means that the observer must be on a collision course with the highlighted bat. Hence, whereas (B) represents the optic flow seen by a stationary observer, (C) represents the optic flow seen by a moving observer that happens to be on a collision course with the highlighted bat. The constant bearing geometry of an incidental collision course thereby simplifies the sensory challenge of identifying candidate targets in attacks on dense swarms. Darker shading and cyan highlights in (B, C) indicate the extent of the swarm and highlight the targeted region within it. All frames shown are from the original video collected by the authors in this study.
